The recording protocol included 40 object classes with 50 images each, taken from the ImageNet dataset , resulting in a total of 2,000 images.
Six participants viewed the visual stimuli in a block-based design. All 50 images corresponding to a single class were shown consecutively in a single sequence. Each image was shown for 0.5 seconds for a total duration of 25 seconds per block/class. After each block/class, a 10-second blank screen (with a black background) was shown, then the next class started. Because of the total time needed for the experiment, its execution was split into four sessions of 10 classes each (about 4 minutes per session). After each session the subjects had time to rest and they continued the experiment whenever they felt ready. Short block durations are critical to keeping participants engaged , ,  and to maximize EEG recording quality . Excessively long block duration is expected to result in extremely low quality data because subjects become inattentive and are likely to create more movement and ocular artifacts. In turn, this would amplify the contribution of the temporal correlation as that becomes the main signal left in the data.
It should be noted that [1]} failed to replicate the block durations from [2]}: experiments in [1]} lasted over 23 minutes, while in [2]} block durations were about 4 minutes.
